1. Field of the Invention
The present invention relates to power supplies for providing precision control of high DC welding currents. The invention relates, in particular, to power supplies for consumable electrode arc welding.
2. Description of the Prior Art
DC welding machines are used in many types of electric arc welding. One common type of DC power supply for such welding machines is the transformer-rectifier type, which utilizes a step-down voltage transformer coupled to an AC supply, with a rectifier connected in the transformer secondary. This type of power supply is used with various consumable electrode arc welding systems, such as metal inert gas ("MIG") . Various methods have beer used to adjust or control the welding current in such a power supply. Typically, control is exercised in the transformer primary, such as by the use of a tapped primary or by primary phase control, to vary the secondary voltage and current. This is done because of the problems encountered in attempting to control the high welding current of the transformer secondary circuit directly. But a tapped transformer winding provides only a step-type control and does not afford continuous adjustment. In primary phase control, the control is continuous but the adjustment is based on the sine wave AC input to the transformer, and the slope of the sine wave is usable only over about one-third of a cycle. Thus, the adjustment provided is non-linear, and is not smooth.
Furthermore, both of these types of transformer primary control also affect the available peak voltage. Thus, in applications such as the welding of thin metals where the power output has to be lowered to prevent burn-through of the workpiece, this also lowers the output voltage available to initially strike the arc, which may create weld problems in the case of poor conductors or dirty metal. Furthermore, the welding control is effected on a 60 Hz AC supply voltage, the frequency of which is low enough that the arc, once struck, can follow the 60 Hz variations and may tend to sputter or quench.
Also, with transformer primary control, the primary is connected to the AC supply only when the welding gun trigger is actuated, but the trigger actuation will be effective only if the trigger mechanism is supplied with power. This requires the use of a control transformer in addition to the power transformer and a contactor or relay responsive to trigger actuation for connecting the primary to the AC supply.
It is known to provide a tapped winding or phase control of the secondary of the power transformer, but the tapped winding still provides a stepped discontinuous control and the phase control is still operating on a sinusoidal waveform and provides a non-linear output with a very small and critical range of adjustment. Furthermore, the tendency to sputtering and quenching remains. Also, such secondary control techniques retain the disadvantage of affecting the peak voltage available.
With respect to the latter problem, some prior welding machines have utilized high frequency control characteristics in the primary of the transformer at a frequency too high for the arc to follow. However, such machines are extremely expensive, since the magnetic components and all of the semiconductors, especially the power semiconductors, must be able to function at the high frequency rate.